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<h3 class="section">23.1 Introduction to Non-Local Exits</h3>

<p>As an example of a situation where a non-local exit can be useful,
suppose you have an interactive program that has a &ldquo;main loop&rdquo; that
prompts for and executes commands.  Suppose the &ldquo;read&rdquo; command reads
input from a file, doing some lexical analysis and parsing of the input
while processing it.  If a low-level input error is detected, it would
be useful to be able to return immediately to the &ldquo;main loop&rdquo; instead
of having to make each of the lexical analysis, parsing, and processing
phases all have to explicitly deal with error situations initially
detected by nested calls.

   <p>(On the other hand, if each of these phases has to do a substantial
amount of cleanup when it exits&mdash;such as closing files, deallocating
buffers or other data structures, and the like&mdash;then it can be more
appropriate to do a normal return and have each phase do its own
cleanup, because a non-local exit would bypass the intervening phases and
their associated cleanup code entirely.  Alternatively, you could use a
non-local exit but do the cleanup explicitly either before or after
returning to the &ldquo;main loop&rdquo;.)

   <p>In some ways, a non-local exit is similar to using the &lsquo;<samp><span class="samp">return</span></samp>&rsquo;
statement to return from a function.  But while &lsquo;<samp><span class="samp">return</span></samp>&rsquo; abandons
only a single function call, transferring control back to the point at
which it was called, a non-local exit can potentially abandon many
levels of nested function calls.

   <p>You identify return points for non-local exits by calling the function
<code>setjmp</code>.  This function saves information about the execution
environment in which the call to <code>setjmp</code> appears in an object of
type <code>jmp_buf</code>.  Execution of the program continues normally after
the call to <code>setjmp</code>, but if an exit is later made to this return
point by calling <code>longjmp</code> with the corresponding <code>jmp_buf</code><!-- /@w -->
object, control is transferred back to the point where <code>setjmp</code> was
called.  The return value from <code>setjmp</code> is used to distinguish
between an ordinary return and a return made by a call to
<code>longjmp</code>, so calls to <code>setjmp</code> usually appear in an &lsquo;<samp><span class="samp">if</span></samp>&rsquo;
statement.

   <p>Here is how the example program described above might be set up:

<pre class="smallexample">     #include &lt;setjmp.h&gt;
     #include &lt;stdlib.h&gt;
     #include &lt;stdio.h&gt;
     
     jmp_buf main_loop;
     
     void
     abort_to_main_loop (int status)
     {
       longjmp (main_loop, status);
     }
     
     int
     main (void)
     {
       while (1)
         if (setjmp (main_loop))
           puts ("Back at main loop....");
         else
           do_command ();
     }
     
     
     void
     do_command (void)
     {
       char buffer[128];
       if (fgets (buffer, 128, stdin) == NULL)
         abort_to_main_loop (-1);
       else
         exit (EXIT_SUCCESS);
     }
</pre>
   <p>The function <code>abort_to_main_loop</code> causes an immediate transfer of
control back to the main loop of the program, no matter where it is
called from.

   <p>The flow of control inside the <code>main</code> function may appear a little
mysterious at first, but it is actually a common idiom with
<code>setjmp</code>.  A normal call to <code>setjmp</code> returns zero, so the
&ldquo;else&rdquo; clause of the conditional is executed.  If
<code>abort_to_main_loop</code> is called somewhere within the execution of
<code>do_command</code>, then it actually appears as if the <em>same</em> call
to <code>setjmp</code> in <code>main</code> were returning a second time with a value
of <code>-1</code>.

   <p>So, the general pattern for using <code>setjmp</code> looks something like:

<pre class="smallexample">     if (setjmp (<var>buffer</var>))
       /* <span class="roman">Code to clean up after premature return.</span> */
       ...
     else
       /* <span class="roman">Code to be executed normally after setting up the return point.</span> */
       ...
</pre>
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